Rubber will not directly bond to metal. The metal must first be painted with primer prior to bonding the rubber. The primer provides a small surface area and low strength bond between the metal and the rubber. Thus, the weak link in the bond is merely the “strength” of the primer. This process is used in products as diverse as car bumpers, armored tank tracks and engine mounts.
For example, armored tank tracks are made of metal machined to form the links and track plates which are fitted together to form a continuous belt. Most tank plates have rubber pads which provide better traction and prevent the roads from being chewed up by the tank tracks. In addition, the “road and bogey wheels” which are found inside the tracks are also generally coated with rubber to increase traction, reduce metal to metal wear, limit vibration in the suspension and reduce the noise the tank makes in motion.
The steel track plates are painted with primer before the rubber is bonded with heat and pressure. The rubber pad can be separated from the track plate by shearing, due to normal operations (acceleration and steering) or rough road surface, and by heat deterioration, due to friction and vehicle weight (approximately 130,000 pounds). A complete set of tracks for a U.S. Army M-1 Abrams tank can cost as much as $100,000.00 and may only last from 300 to 2000 miles.
In another example, a car bumper is made of rubber coated steel. The steel backing is painted with primer. The rubber is then bonded to the primed steel with heat and pressure. The rubber coating may stand up to straight on (perpendicular) forces, but will easily be “peeled” or “sheared” off by forces from the side (lateral forces). This peeling may even occur, and thus the car bumper will fail, before either the steel backing or the rubber has worn.
Aside from the paint-like primer method, other methods have been used to increase the surface area and strength of the bond between metal and rubber. They fall into three categories: mechanical or chemical etching, machining or channeling, and perforation. Etching consists of abrasive, shot or bead blasting and selective surface erosion by exposure of the metal to acids or strong chemical solutions. Machining or channeling involves the use of deeper cuts and bends in the metal. Perforation allows the rubber to penetrate and form plugs which resist delamination behind the metal. Each of these methods are expensive, and each method weakens the metal surface and may concentrate delamination forces within the rubber compound, thus defeating their purpose.
In a completely different field, inventors have proposed open-celled foams made of metal or the like for use as lightweight building materials, solid propellant reinforcement and burning rate modifiers, battery plates, fluid phase separators, heat shields, heat exchanger cores, radiation shields, fluid filters, shock absorbers, as well as in numerous other applications.
Walz, Reticulated Foam Structure, U.S. Pat. No. 3,946,039, Mar. 23, 1976, describes the process by which a reticulated foam structure is made of metals, ceramics, polymers, etc. In Walz' method, an original polyurethane foam, or sponge, is manufactured to the desired specification of the metal foam which is desired. Then a fluid suspension of a metallic salt is introduced into the original polyurethane foam structure and allowed to set to a rigid structure. This step is called the investment. In this way, a “positive” is formed of the original polyurethane foam structure. The next step is the removal of the original polyurethane foam structure, so as to provide a pattern of voids or internal passageways in the investment which correspond to the original foam structure. In the next step, molten metal is poured into the positive which fills the voids of the positive, forming the final reticulated foam structure which is nearly identical to the original polyurethane foam. Finally, the investment is dissolved in a convenient medium, leaving a metal foam with all the pores empty.
Under this process, reticulated foams may be prepared of various metals, such as aluminum, steel, beryllium, magnesium, uranium, iron, etc.; alloys, such as aluminum-silicon, aluminum-magnesium, and aluminum-zinc; ceramics based on aluminum oxide, silicon dioxide, ferric oxide, including refractories, such as carbides and nitrides; and organic polymers, such as polymides, polyaromatic ethers and thioethers, fluorocarbons. The pore sizes of the inorganic composition vary from 3 to 125 pores per linear inch (ppi). Commercially, pore sizes may be obtained in at least a range of 10 ppi to 100 ppi.
Walz, Method of Making an Inorganic Reticulated Foam Structure, U.S. Pat. No. 3,616,841, Nov. 2, 1971, is substantially similar. Others have improved on the process, suggesting use of various materials for the original foam, such as natural reticulated materials like sponges and coral.